1. Field of the Invention
This invention relates generally to protection of electronic systems against electrostatic discharge. The invention relates specifically to electronic means for protecting communication pads covered by a dielectric layer as is used in proximity communication systems.
2. Related Art
Proximity communication is an I/O technology that allows two chips in face-to-face alignment to communicate without wires, as Drost et al. have explained in “Proximity Communication,” IEEE Journal of Solid-State Circuits, vol. 39, no. 9, September 2004, pp. 1529-1535. In the most widely used implementation, corresponding arrays of electrode plates or pads are formed in the opposing surfaces of the two chips, which are then fixed together with a dielectric layer in between to form a large number of capacitively coupled links between the chips. The pads are electrically conductive and typically formed of a metal. An exemplary structure illustrated in the cross-sectional view of FIG. 1 includes an upper chip 12 and a lower chip 14 having respective electrical pads 16, 18 formed in their active surfaces. Although only one pair of pads 16, 18 is shown, a proximity communication system typically includes a large number of paired pads 16, 18 closely packed in a one- or two-dimensional array. Respective thin dielectric layers 19, 20 cover the active surfaces of the chips 12, 14 and their pads 16, 18. The dielectric layers 19, 20 may be conventional passivation layers covering an integrated circuit. The chips 12, 14 are brought into juxtaposition such that the pads 16, 18 face each other and form between them a capacitor having the intermediate dielectric layers 19, 20 acting as the gap of the capacitor. This structure provides a capacitive coupling circuit between the chips 12, 14. A driver 22 in the upper chip 12 is connected to its pad 16 and impresses an electrical data signal on the pad 16. A receiver 24 in the lower chip 14 is connected to its pad 18 and senses an electrical data signal impressed on the pad 18 from the other pad 16. Proximity communication is particularly effective in being able to provide a large number of paired pads in closely spaced one- and two-dimensional arrays to provide a wide communication bus between the chips 12, 14. The communication bus may operate in different directions for different paired pads.
Electrostatic discharge (ESD) is a well known effect in which electrical charge arising from extraneous sources is somehow impressed on electronic circuitry and may induce signal levels which the circuitry is not designed to withstand. ESD events have the potential to destroy transistors or other chip elements connected to a proximity communication pad. The conventional method of protecting against ESD events would be to add a parallel path that could dissipate the energy of an ESD event within a few nanoseconds, and hence protect sensitive circuit elements, acting akin to a lightning rod on a building. Unfortunately, this approach is impractical for proximity communications because it would add so much parasitic capacitance to the critical proximity pad that the received signal would be unacceptably attenuated.
The signal levels of proximity communication systems are very low, and the sensitive circuitry used to detect these levels may be prone to ESD failure. However, the magnitude of electrostatic discharge in capacitively coupled proximity communication has not been quantified, and solutions to ESD have not been widely promulgated.